In fiber-optic communication, an optical add-drop multiplexer (OADM) is an optical node used in wavelength-division multiplexing (WDM) systems for multiplexing and routing different channels of light into or out of a single mode fiber. Typically, an OADM has the capability to add one or more new wavelengths to an existing multi-wavelength WDM system, and to drop, or remove, one or more wavelengths from the multi-wavelength WDM system by passing the dropped signals to another network path. A reconfigurable optical add-drop multiplexer (ROADM) is a type of OADM that enables an operation to remotely switch traffic from a WDM system at the wavelength layer without manual intervention.
The complexity of ROADMs in optical networking continues to increase, and has placed considerable strain on the requirements for switching elements such as wavelength selective switches (WSSs). However, increases in the number of ports and other features supported by a 1×N WSS impact isolation performance, and multiple switches become necessary to meet channel isolation requirements in ROADM. For example, broadcast and select architectures are expected to migrate to route and select, doubling the number of WSSs required on the line side of the ROADM, increasing complexity and cost. Architectures that provide a way to reduce the number of switching elements per ROADM degree are therefore of particular interest.
On the other hand, relentless growth in communications traffic continues to challenge optical network capacity. Additional bands of spectrum need to be considered eventually as a means for increasing transmission capacity per fiber. Indeed, early work on more extreme approaches such as the use of multi-core fibers is underway. The use of multiple spectrum bands conserves fiber assets, which is particularly important where fiber is scarce, where premium fibers are deployed and used, and where fibers are leased.
In optical networks today a single-band approach, e.g., C band or a somewhat wider “extended C band”, is preferred, particularly for ROADM-centric architectures. The addition of further bands is possible, but the deployment of an essentially parallel set of equipment for one or more other bands, e.g., L band, on an existing fiber is typically less attractive than the deployment of a parallel set of C band equipment on a new fiber, especially when the fiber is readily available. For example the equipment required for other bands is often more expensive or commercially unavailable. Viable architectures that reduce the cost of multi-band equipment in comparison to equivalent single-band solutions are expected to become very important but are not yet available.